Known bindings for cross-country skis in which the front of the boot is attached to the ski and the heel pivots around an axis transverse to the longitudinal axis of the ski, can be classified into two distinct categories. The first, called hinge-type bindings include boot bindings in which the axis is stationary and is separated from the boot as in French patent. Nos. 2,096,002, No. 2,200,026, No. 2,439,602 and No. 2,447,731. This category also includes bindings in which the axis is integrated into a front extension of the boot as in German Patent No. A1-3,015,052, and bindings in which the axis is "virtual", its location being determined by a zone of least rigidity formed by a reduction of the thickness in the transverse direction in a front extension of the boot, as shown in French patent No. 2,306,721.
The second category includes a "flexion" device which bends when the heel is raised. This "flexion" device may comprise an elastic element whose front section is attached to a ski and whose rear section is attached to the toe of a boot. Alternatively, the elastic element itself may comprise a front extension of the toe of a boot. In either case, the elastic element progressively bends during the lifting of the heel, which causes a pivoting of the foot around an instantaneous rotation axis whose angular orientation changes at the same time as the heel pivots. This type of binding is disclosed in French patent No. 2,447,731.
These bindings have led to a great improvement in the stride of the skier by avoiding excessive bending of the boot at the level of the toes. In these bindings the bending is taken into account by the hinge or the flexible element and by encouraging the joints of the foot and of the leg to perform useful work. The stationary or instantaneous pivoting axis that is found in all of the above-mentioned and all equivalent patents is positioned on the surface of the ski and is perpendicular to the ski's longitudinal plane of symmetry, a position which appeared to be the easiest and most appropriate for the skier. However, if one places oneself along the axis of the ski trail, behind a cross country skier, one notes that the heel of the skier, during the lifting of the foot, describes a trajectory which deviates from an ideal vertical plane passing through the trail. Furthermore, one notes that the ski tends to undergo either a large or small rotation around its longitudinal axis. As a result, the ski tends to move away from its optimal position in relation to the trail. The skier responds by unconsciously making an opposing, supplementary effort. This phenomena is due to the bony anatomy of the leg of the skier which is obviously impossible to change.